As primary human cells divide in culture or in vivo with aging, telomeres shorten due to the strict down regulation of telomerase and its catalytic subunit, TERT. Continued proliferation leads to replicative senescence, a block to continued cell growth. Inactivation of RB and p53 can bypass senescence, allowing further cell division and telomere shortening that triggers crisis, a state of massive cell death and genomic instability. The previously accepted model predicted that senescence and crisis were both tumor suppressor mechanisms that limit the replicative potential of primary cells. This theory is supported by the observation that all fully transformed cancer cells have reestablished telomere maintenance, either by reactivation of telomerase or activation of telomerase-independent mechanisms (ALT). Using mice that are deficient for both telomerase and p53, I present preliminary data showing that cell death during early crisis is p53-mediated, but that cells die in late crisis by p53-independent mechanisms as a consequence of rampant genomic instability. In contrast to the predicted model, tumorigenesis is accelerated in mouse cells that lack p53 and exhibit telomere dysfunction. These results indicate that crisis is not a tumor suppressor mechanism, and instead may represent a period during early tumor development in which marked genomic instability facilitates carcinogenesis. The specific aims of this grant are directed toward an in vivo validation of this new understanding of crisis and a dissection of mechanisms by which telomere dysfunction leads to chromosomal instability and cancer. The specific aims are: (1) To determine the impact of telomere dysfunction and concomitant genomic instability on the cancer phenotype in mice with impaired p53 function (2)To characterize the adaptive responses that allow tumor cells to evade telomere checkpoints, including analysis of telomere dynamics, chromosomal fusions, cytogenetics by SKY and modulation of DNA damage pathways (3) To elucidate differences in cell type specific responses to telomere dysfunction in a transgenic breast cancer model. Transplantation experiments using this model will reveal if the effect on tumor biology is a cell autonomous one.